1. Field of Invention
The present invention relates generally to inverters. More particularly, at least one embodiment of the invention relates to methods and apparatus for improving the performance of an inverter and for providing protection for components of the inverter.
2. Discussion of Related Art
Inverters are used in a variety of different power systems. For example, inverters are commonly used in uninterruptible power supplies (UPSs) to provide regulated, uninterrupted power for sensitive and/or critical loads, such as computer systems and other data processing systems. In at least one type of a UPS, an inverter has a battery state of operation and a charger state of operation. The battery state of operation is entered when the alternating current (AC) power provided to a load by a main AC input is not able to support the load. In the battery state of operation, the inverter converts direct current (DC) power from a battery into AC power, and provides the AC power to the load. When the main AC input is able to support the load, the inverter enters the charger state of operation. In the charger state of operation, the inverter converts AC power from the main AC input into DC power and supplies the DC power to the backup battery to charge the battery.
In certain power systems in which single-phase inverters are used, a load may be connected to multiple different phase lines and to a neutral line. An example of such a system is illustrated in FIG. 1. In the illustrated example, the power system 100 includes a three-phase supply system having a common neutral line 102, three power lines/phases 104, a plurality of loads 106 coupled to the three phases 104 and to the neutral line 102, and an inverter 110 coupled to one of the three phases 104 and to the neutral line 102. In some circumstances it is possible that the neutral line 102 or one of the phases 104 may fail, for example, due to a blown fuse or other condition. For example, point 108 illustrates a fail or open condition on the neutral line 102. A failure in the neutral line 102 (or one of the phases 104) can cause an imbalance in the loading of the power system 100, a condition referred to as dynamic loading, which causes unpredictable voltages to be provided to the inverter 110 and the plurality of loads 106 connected to the power lines. In particular, the input voltage to the inverter 110 may sag or swell unpredictably, causing the inverter to fail.